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Old 1st December 2004, 03:38 PM   #1
larryg is offline larryg  United States
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Default leach amp dc off-set

i have my new leach amp up and running. bias is set to 100ma.
in checking the dc off-set the left ch. measures .030 and the right
ch. measures .054. i did not match the transistors or the zener
diodes. is this difference normal? any help will be greatfully appreciated.
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Old 1st December 2004, 04:07 PM   #2
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Hello,

30-50mV is ok for DC

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Old 1st December 2004, 04:33 PM   #3
larryg is offline larryg  United States
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thanks Jen Rasmussen!
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Old 1st December 2004, 04:53 PM   #4
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Mornin,
If you're really concerned about this small difference, take a look at D 13,14,15,16. D13 & 14 need to match D 15,16 fairly close.

From the Leach page,
"The bias currents in the diff amps are regulated by zener diodes. In each diff amp, two 20 V diodes are used in series to form an equivalent 40 V zener. Although a single diode could be used, the error tolerance is less when two diodes are used in series. I prefer to measure the zener voltage of each and pick series combinations of diodes that give equal reference voltages on each circuit board. It is not necessary to have exactly +40 V and -40 V. But the two voltages on each circuit board should be as close as possible to minimize dc offset problems.

The zener voltage of a diode can easily be measured by connecting a resistor in series with it on an electronic breadboard and connecting a bench power supply across the series combination. The power supply voltage should be set so that the current through the diode is 3.3 mA. If you don't have a current meter, the voltage across the resistor should measure 3.3 x R, where R is in kohms. For example, the voltage across a 2 kohm should be 6.6 V. Once the current is set, the dc voltage across the zener diode can be measured with a voltmeter. For example, if you have +15 V and -15 V supplies, connect the diode in series with a 3 kohm resistor across the two outputs and measure the diode voltage. If you get the diode backward, the resistor will get hot."

And a big hurrah getting it up and running. Playing any real music yet? Folks will be happy to see your pics.

Prosit
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Old 1st December 2004, 06:29 PM   #5
larryg is offline larryg  United States
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thanks acenovelty! looks like good info to me! yes i have been
listenning to the amp! tryed it this morning for the first time.
man was i happy!! hope to post pics soon.
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